This invention relates to a constant-current output circuit for outputting a constant current to a cable.
The IEEE 1394 standard defines differential data transmission making use of a twisted-pair cable.
U.S. Pat. No. 5,418,478 discloses a CMOS differential circuit for driving a twisted-pair cable. This circuit is provided with a first PMOS drive transistor having a drain electrode coupled to a first signal line of a cable, a second PMOS drive transistor having a drain electrode coupled to a second signal line of the cable, a first NMOS drive transistor having a drain electrode coupled to the first signal line, and a second NMOS drive transistor having a drain electrode coupled to the second signal line. When the first PMOS drive transistor pours electric current into the first signal line, the current, brought back by way of a terminating resistor and the second signal line, is drawn into the second NMOS drive transistor. When the second PMOS drive transistor pours electric current into the second signal line, the current, brought back by way of the terminating resistor and the first signal line, is drawn into the first NMOS transistor. In other words, the first and second PMOS drive transistors form respective constant-current output circuits for outputting a positive constant current to the cable, and the first and second NMOS drive transistors form respective constant-current output circuits for outputting a negative constant current to the cable.
Then, it is designed such that each of the two signal lines of the cable is coupled, through a terminating resistor Rt, to a bias voltage Vm. Here, Vdd and Vss are power supplies for the constant-current output circuits. The power supply Vdd provides a fixed power-supply voltage (for example, a positive voltage of +2.5 V, to each of source electrodes of the first and second PMOS drive transistors. The power supply Vss provides a fixed power-supply voltage, (for example, a ground voltage of 0 V) to each of source electrodes of the first and second NMOS drive transistors. The bias voltage Vm of the cable, viewed from the power supply Vdd, varies according to the potential of equipment that is connected to the other end of the cable. Likewise, the cable bias voltage Vm, viewed from the power supply Vss, varies according to the potential of the other cable end equipment. For example, in the case the ground voltage of the other cable end equipment is higher than the voltage of the power supply Vss of a constant-current output circuit in point, the bias voltage Vm, viewed from the power supply Vdd of that constant-current output circuit, becomes smaller. If the drain-source voltage of each of the first and second PMOS drive transistors becomes too small due to such variation in the bias voltage Vm, then the operation points of these transistors each move from the saturation region to the linear region as long as their gate-source voltage is maintained at a fixed value, as a result of which constant current output will no longer be maintained. Conversely, in the case the ground voltage of the other cable end equipment is lower than the voltage of the power supply Vss of a constant-current output circuit in point, the bias voltage Vm, viewed from that power supply Vss, becomes smaller. If the drain-source voltage of each of the first and second NMOS drive transistors becomes too small due to such variation in the bias voltage Vm, then the operation points of these transistors each move from the saturation region to the linear region as long as their gate-source voltage is maintained at a fixed value, as a result of which constant current output will no longer be maintained. Further, there may be a case in which the bias voltage Vm of the cable is intentionally changed for the communication of data transmission rate set information and electric power management information, and also in such a case, the same problem arises.
Accordingly, an object of the present invention is to provide a constant-current output circuit capable of maintaining a constant current drive capability, regardless of the variation in cable bias voltage.
In order to accomplish the object, the present invention provides a constant-current output circuit for outputting a constant current to a cable coupled, through a terminating resistor, to a bias voltage. In this constant-current output circuit, a drive transistor for outputting a current to the cable is provided, and when a change in bias voltage of the cable causes the drain-source voltage of the drive transistor to vary, a variation in drain current of the drive transistor accompanied by such voltage variation is compensated and the current drive capability of the drive transistor is adjusted so as to establish a substantial correspondence between a drain current of the drive transistor and the constant current. Adjustment to the current drive capability is made by controlling, for example, the gate or substrate voltage of the drive transistor. Such adjustment is carried out on the basis of a result of the detection of changes in bias voltage of the cable or on the basis of the system information indicative of a prediction of the change in bias voltage of the cable.
The drive transistor in the constant-current output circuit of the present invention can be replaced by a driver formed by a plurality of drive transistors connected together in parallel. Adjustment to the current drive capability of the driver is made by digital control of the number of transistors which are subjected to activation or by analog control of the gate voltage of a particular transistor.